A method of measuring the permeability of hollow fiber membranes by means of a transient diffusion experiment was developed and demonstrated in our previous work. One goal of the proposed research is to make this method more efficient and flexible by developing (1) a semi-continuous concentration monitoring system which is compatible with several methods of concentration measurement and (2) a procedure which requires the preparation of only one membrane sample per solute. A second goal is to use the transient diffusion experiment to study diffusion in blood at mass-transfer rates comparable to those encountered in the artificial kidney. The effects of binding to plasma and red cell proteins and disequilibrium between phases on diffusion in blood will be assessed by studying the diffusion of such solutes as urea, creatinine, and uric acid through various blood components enclosed by sealed hollow fiber membranes.